For offsets in drain stacks, where must the vent be located?

• A. Immediately above the offset
• B. At the bottom of the offset
• C. Where five or more branch intervals are located above the offset
• D. There is no venting requirement

Answer: (C)

When a drain stack has an offset, the vent must be placed in a position that can affect the portion of the stack above the offset. The rule in many codes is that once there are five or more branch intervals above the offset, the vent must be located above the offset. This ensures air can reach the vertical portion of the stack as it drains, preventing negative pressure and siphoning of traps in the upper fixtures. If the vent were placed immediately above the offset, it wouldn't adequately vent the higher branches; placing it at the bottom of the offset or not venting at all would fail to protect the upper portions of the stack.